For single pass type printing of substrates, printing systems are known to comprise elongate lineheads with stationary rows of inkjet nozzles. For larger widths, such lineheads each comprise an elongate support bar that is equipped with a plurality of printheads, of which each printhead is replaceable and comprises a number of the nozzles. It is of great importance for the image quality that can be obtained with printing on a substrate, that each printhead is accurately positioned, both relative to printheads of their own linehead as well as relative to printheads of other lineheads. Dimensional stability of the nozzle locations in the printheads in the printing direction x as well as in a direction y perpendicular thereto is crucial. Another important aspect is that the support bar needs to have an expansion behaviour that is matched to the printheads, and any intermediate connection elements therebetween, during changes of temperature. This is important in order to prevent that, transitions between respective printheads become visible on a printed substrate if a pitch between two nozzles of two adjacent printheads becomes different from a pitch between two nozzles of a same printhead. This is also important because, in the case that each printhead is mounted to the support bar with two or more interspaced mounting positions in the y-direction, the support bar may start to curve in the case of such temperature changes. Moreover, considering two or more lineheads from the same system may have different temperatures but the nozzles on these lineheads have to stay aligned, it is often preferred that the support bar have a low thermal expansion coefficient and high thermal conduction. Furthermore, considering the linehead may span a large width, it is preferred that the support bar has a high e-modulus and is lightweight.
For example US 2013/0265363 shows a printing bar unit which comprises a T-shaped support bar which at both sides of a vertical portion is provided with four engagement recesses. Each recess can receive a complementary engagement projection of a printhead, substantially with a form fit. Each recess furthermore is provided on its opposite edges with tapped holes for mounting one of the printheads thereto. For this each printhead includes a printhead body that is able to eject ink from an array of inkjet nozzles, and a fixing member. The fixing member is pre-mounted to the printhead body with screws. A horizontal portion of the T-shaped base plate is provided with communication holes which are connectable to ink channels of the printheads. The communication holes connect to an ink supply tube. Inside the printheads the inkjet nozzles are each equipped with a controllable piezoelectric element.
A disadvantage herewith is that the printing bar unit is difficult and expensive to manufacture, particularly if the unit needs to span large printing widths, as for example may be the case when it is to be used as elongate linehead for single pass type of printing, in which the linehead needs to span the entire width of substrates to be printed. Furthermore it is disadvantageous that a high positioning accuracy of each of the individual printheads relative to the support bar is strongly dependent on the accuracy with which the support bar itself is manufactured and is strongly dependent on the rigidity of the support bar during use, for example when heating of the unit may occur.
GB-2,449,939 discloses a method for manufacturing a printhead support, in which an elongated support member is provided with connecting apertures that are positioned approximately where corresponding printhead alignment members are to be located. This support member gets placed on top of a jig in such a way that the connecting apertures get positioned around accurately located upwardly projecting spigots of the jig. With this the connecting apertures have a larger diameter than the corresponding spigots. Gaps between the spigots and the connecting apertures then get filled with a hardenable material. As soon as this material has hardened, the support member gets removed from the jig leaving mounting apertures behind where the spigots were located. Those mounting apertures then are destined for having printheads mounted thereto with their printhead alignment members.
A disadvantage with this is that the support member needs to be held securely in place on the jig not only during filling of the gaps with the hardenable material but also during hardening thereof. The slightest movement between the support member and the jig, immediately deteriorates the positioning accuracy of the to be formed mounting apertures. Another disadvantage is that for the hardening of the material, heating and/or curing is necessary, which may cause the support member and/or the jig to expand/deform, which then immediately may have a negative effect on the positioning accuracy of the to be formed mounting apertures. Yet another disadvantage is that measures need to be taken in order to prevent the hardenable material from sticking to the jig. Further it is noted that with this method the degree of positioning accuracy may leave to be desired, for example due to variations or deviations in shrinkage of the hardenable material during hardening. Also it is noted that the hardenable material needs to be from a specific type that is able to harden to a sufficient high and accurate degree, like Diamant Moglice, which makes it relative expensive.
There also needs to be access to insert the hardenable material whereas the geometry not always permits to have insertion holes and excess spill apertures. Finally it is noted that the filling of the gaps with the hardenable material is a relative difficult and time-consuming operation which is likely to contaminate the support member at locations around the connecting apertures.